1. Field of the Invention
The present invention relates to a laser processing method and a laser processing apparatus to perform laser processing for a workpiece such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device manufacturing process, plural areas are partitioned by division lines arranged in a lattice manner on a surface of a semiconductor wafer having a substantially circular disc shape and devices such as ICs and LSIs are formed in these partitioned areas. Then, the semiconductor wafer is cut along the division lines to separate the areas in which the devices are formed, so that the individual devices are manufactured. As a method for dividing the above-described semiconductor wafer along the division lines, a laser processing method is attempted in which a pulse laser beam having such a wavelength as to be transmitted through the wafer is used and the wafer is irradiated with the pulse laser beam with the focal point positioned inside the area along which the dividing should be performed.
The dividing method using this laser processing method is the following technique. Specifically, a modified layer is continuously formed inside the wafer along the division lines by irradiating the wafer with the pulse laser beam having such a wavelength as to be transmitted through the wafer along the division lines with the focal point of the pulse laser beam positioned at the inside corresponding to the division line from one surface side of the wafer. Then, the wafer is divided into the individual devices by applying an external force along the division lines whose strength is lowered due to the forming of this modified layer (refer to e.g. Japanese Patent No. 3408805). The above-described laser processing technique is to continuously form the modified layer inside the wafer along the division lines by irradiating the wafer with e.g. a pulse laser beam with a wavelength of 1320 nm having transmissibility to a silicon wafer along the division lines with the focal point of the pulse laser beam positioned at the inside corresponding to the division line, and has an advantage that the width of the division line can be made small.